Coriolis Vibratory Gyroscopes (CVGs) are useful for measuring or detecting angular movement or acceleration and are based on vibrating mechanical elements which sense rotation. A resonator of a CVG is excited into vibration at its resonant frequency. The direction or orientation of vibration is configured so that it couples with the rotation of the sensor allowing the vibration to be analyzed to sense directional changes. A readout and control system is used to apply excitation energy to the resonator of the CVG and to sense the resulting vibration. A number of circuits are employed to excite the resonator to resonance, sense the vibration and to produce an output indicative of rotational rate.
In certain CVGs, a readout circuit extracts the charge from one or more capacitive pickoffs of a vibratory gyroscope to measure the amplitude of the resonator vibration. The resonator vibration responds to the amplitude of a drive signal supplied to one or more forcer electrodes to induce or modify the vibration in the gyroscope resonator. Capacitive pickoffs may be used to measure the vibratory displacement of the resonator through the electrical charge they produce. The charge from the capacitive pickoff(s) is transferred to an integration capacitor (CF) or feedback resistor (RF) where the charge is converted to an output voltage. That output voltage is sampled and, for the anti-nodal channels, is used to measure the amplitude of the resonator vibration. A controller adjusts the drive amplitude to maintain the measured vibration amplitude at a predetermined value. Also, by measuring both in-phase and quadrature components of the anti-nodal signal, it is possible to determine frequency error (i.e. deviation between the excitation frequency and the resonant frequency). A controller can then use this information to correct the frequency and ensure the drive tracks the resonant frequency of the CVG.
In one example of a CVG, a typical resonator can be composed of a thin shell of fused quartz having a hemispherical geometry. The shape of the resonator can be specified by specifying the diameter or the outer shell surface, the inner shell surface, with the shell surfaces having a common sphere center and the shell having a uniform thickness. Eight electrodes can be constructed on a partial hemispherical dome surface residing inside the resonator, and 32 electrodes can be constructed on the inside of a partial hemispherical cavity residing outside the resonator. The inner electrodes are used as pickoffs and the outer electrodes as forcers. The forcers provide the method for controlling the amplitude and location of the standing wave of the resonator. That is they hold the standing wave pattern at a given case position by applying a force proportional to the rotation rate of the gyro, they provide a force to keep the amplitude of the standing wave at a desired amplitude and they provide the forces that keep a standing wave in quadrature to the desired state from developing.
The feedback signals to the forcers from the gyro control loops are each of different frequency content which helps decouple the loops, thus minimizing cross-talk between the control loops. The ‘rate’ servo is controlled with an AC signal at a frequency, omega, synchronous to the standing wave. The ‘amplitude’ servo uses a signal at 2*omega which drives what is termed the ‘parametric’ drive. The ‘quadrature’ loop keeps a traveling wave from developing using DC signals to balance the frequencies of the two gyro modes. In order to decrease the size of this design the outer electrodes can be eliminated, and the gyro control method modified so that the gyro utilizes only the 8 inner electrodes for both pickoff and forcer functionality. To operate with this reduced set of electrodes the gyro needs to be mechanized using one-omega signals for all three of the control loops. The result is that control loop cross-coupling creates performance issues for the gyro.